Add HF Spaces thumbnail and short_description for social sharing

- thumbnail: Custom preview image for social media cards
- short_description: 'A Holistic Benchmark for Software Engineering'
- Updated colors to yellow/yellow to match OpenHands brand

Co-authored-by: openhands <openhands@all-hands.dev>

README.md

@@ -1,8 +1,8 @@
 ---
 title: OpenHands Index
 emoji: 🤖
-colorFrom: blue
-colorTo: purple
+colorFrom: yellow
+colorTo: yellow
 sdk: docker
 app_file: app.py
 pinned: true
@@ -12,6 +12,8 @@ app_port: 7860
 failure_strategy: none
 tags:
   - leaderboard
+thumbnail: https://huggingface.co/spaces/OpenHands/openhands-index/resolve/main/assets/openhands-index-preview.jpg
+short_description: A Holistic Benchmark for Software Engineering
 ---
 
 ## OpenHands Index Leaderboard